Hi Ruzzy,
You don’t have to imagine anything when you practive WILD. The main techniques are counting, repeating mentally a sentence or focusing on something (breathing, part of your body, etc.) until you reach the hypnagogic state.
About VILD, you’ll find all the explanation there:
The BIG VILD topic
Visualisation skills are necessary for VILD’ing.
It depends on people. If you can reach the hypnagogic state very quickly, it could take few time. If you don’t, you better have to learn another technique (MILD, autosuggestion, RC’s).